1. Field of the Invention
This invention relates to an arc stud welding machine having an arc shield mount section for retaining a generally semicylindrical arc shield segment, and more particularly to an arc stud welding machine comprising two levers each having an arc shield segment mounted at the tip thereof, said levers being openable and closable for contacting and separating the two arc shield segments.
2. Description of the Prior Art
In arc stud welding, a welding accessory called an arc shield (ferrule) has conventionally been used for molding the shape of the molten metal, shielding the weld portion from external air, and other such purposes.
Such arc shields have in the past been made of, for example, ceramic. The conventional ceramic arc shield breaks under the thermal shock (1,300.degree. C. and higher) during welding and cannot be reused. Therefore after welding it is removed from the weld portion by smashing it with a hammer or the like.
Thus the conventional ceramic arc shield entails the problems that it is an expendable item which must be employed one for each stud and cannot be reused and, moreover, that it must be fitted one per stud and then, after welding, be smashed for removal.
In order to overcome these problems, it has been proposed for example in Japanese Unexamined Patent Publication No. 59(1984)-141379 and Japanese Unexamined Utility Model Publication No. 60(1985)-181269 that the arc shield be made reusable a number of times by constituting it from a material with superior thermal shock resistance property and forming it of separable segments.
That is, it has been proposed that the arc shield be made reusable a number of times by constituting it from a material with superior thermal shock resistance property and forming it of separable segments, making the segmented arc shield (i.e. the arc shield segments) capable of contacting with and separating from each other, carrying out stud welding when the arc shield segments contact to form a cylindrical arc shield, and removing the arc shield from the welded stud by separating the arc shield segments after completion of welding, thereby enabling repeated use of the arc shield numerous times.
In the case of fabricating the arc shield of a material superior in thermal shock resistance and in a segmented form so as to make the arc shield repeatedly usable a large number of times, it is conceivable to use an arrangement wherein, for example, the arc shield is fabricated of a ceramic having superior thermal shock resistance property, the welding machine is provided with a pair of arc shield mount sections, each of the arc shield mount sections is caused to retain one generally semicylindrical arc shield segment of the two segments into which the arc shield is divided, the two mount sections are brought toward each other to form a cylindrical arc shield from the two arc shield segments, arc stud welding is thereafter conducted and, after the completion of the welding, the two mount sections are separated, whereby the arc shield is removed from the welded stud.
It is further conceivable to realize the clamping of the arc shield segment by the arc shield mount section by, for example, gripping the generally semicylindrical arc shield segment between a similarly shaped approximately semicylindrical inner clamp member and an outer clamp member.
However, the arc shield segment is, as mentioned above, made of ceramic and has an extremely low coefficient of thermal expansion while the arc shield mount section, including the inner and outer clamp members, would generally be made of metal and have a high coefficient of thermal expansion, so that in applying the aforesaid arrangement if the arc shield segment is simply clamped by the arc shield mount section having a different coefficient of thermal expansion, the high temperature at the time of welding would cause thermal stress due to the difference in coefficients of thermal expansion to act on the arc shield, thus making the arc shield vulnerable to breakage.
Further, in the above-described welding machine, the arc shield segments are contacted with each other by pressing the welding machine onto the base metal against the force of a compression spring and are separated from each other by the force of restoration of the compression spring when the force pressing the welding machine onto the base metal is relieved. With this method of contacting and separating the arc shield segments it is difficult to appropriately control the arc shield segment contacting and separating operation. For example, although the separation of the arc shield segments should be conducted by gradually relieving the pressure, it is extremely difficult by a manual operation to relieve the pressure once applied in an appropriate gradual manner and it frequently happens that the pressure is relieved rapidly. This leads to a sudden separation of the arc shield segments, making it difficult to always carrying out the welding operation appropriately and stably.
Further, in the above-described welding machine, no consideration whatsoever is given to such matters as making the welding machine easier for the operator to hold when conducting the welding operation or making the welding operation easier to conduct. The disclosed welding machine is thus poor in welding operability.